This invention relates to circuits which compensate for temperature sensitive variations in device parameters such as, for example, the gain of an avalanche photodiode.
Both PIN photodiodes and avalanche photodiodes (APDs) are being considered as detectors for fiber optic communication systems. The low noise carrier multiplication of an APD increases receiver sensitivity, compared to an identical receiver using a PIN diode, by amount ranging from 10 to 15 dB of optical power, depending on details of the amplifier design, bit rate, etc. This enhancement of sensitivity, however, is achieved at the expense of having to accurately control the bias voltage of the APD and is complicated by the fact that the APD gain versus voltage relationship is temperature dependent.
The dependence of the gain of a typical n.sup.+ p.pi.p.sup.+ APD on bias voltage for several different temperatures is shown in FIG. 1. The gain at a given temperature increases rapidly with increasing reverse bias at low voltages (in this case at approximately 50V) corresponding to sweepout of the device, and then increases more gradually until avalanche breakdown is approached where the gain also increases very rapidly. If the temperature of the device is increased, the gain for a given bias voltage decreases because of the negative temperature dependence of the ionization coefficients for holes and electrons. In order to maintain a constant gain as a function of temperature it is thus necessary to vary the bias voltage of the device in a controlled manner. Failure to do so can cause the APD to be driven into breakdown where it becomes exceedingly noisy, thereby drastically reducing the signal to noise ratio of the system.
In an effort to compensate for the temperature dependence of the APD gain versus voltage relationship, a number of techniques have been proposed in the prior art. One technique devised by L. E. Drew employs a second APD as a temperature dependent voltage regulator. In this technique the APD used to detect the light is operated at a voltage somewhat below the breakdown voltage of the reference APD. Provided the two APDs have similar temperature dependences, the detecting APD will operate at approximately constant gain. Possible difficulties with this technique are the need to use two APDs and to match the APD characteristics. Another technique, referred to as "full AGC" by P. K. Runge, employs feedback in which the electrical signal, proportional to the multiplied photocurrent, is detected and used to control the bias to the APD. In this method the output signal, and hence the APD gain, are held current -- with constant light input -- for aribitrary temperature changes. An added feature is that the output signal is held constant even if the intensity of the light is varied. A potential disadvantage of this type of control is the possibility of instabilities associated with the feedback system. Such instabilities in the form of low frequency motorboating have been observed when the input light signal is removed from the detector.
A variant of the above scheme employs a constant current power supply for the APD. Provided the APD current is proportional to the APD gain and to the input light signal, this scheme is essentially equivalent to that described above. However, if APD leakage currents are appreciable compared to the signal current, then their temperature dependence can result in nonideal operation.
A third method, more complicated than the above, detects the noise output of the device and uses this noise to control the bias and hence the gain of the APD. J. A. Raines et al., S.E.R.L. Technical Journal, Vol. 20, No. 1 (1970). The cost of implementing such a system, however, makes it unacceptable for presently contemplated fiber optic applications.